Asynchronous Transfer Mode (ATM) is a cell switched technology for transferring data over high speed digital communication networks. Unlike Synchronous Transfer Mode (STM) used in today's telecommunication networks, which uses static time division multiplexing to carry multiple connections over a physical transmission link, ATM allows data cells of multiple connections to dynamically share time slots of a transmission link. This dynamic sharing of transmission bandwidth increases a network's efficiency in carrying variable bandwidth traffic, but it also makes it difficult to provide guaranteed quality of service, or QoS, to connections due to contention from multiple connections for the same transmission bandwidth. One commonly used QoS measurement is the end-to-end cell transmission delay. To provide this type of QoS guarantee, a connection must transfer all cells generated at a source to a destination within a user requested delay bound. End to end cell transmission delay bound is usually needed for transferring real-time traffic, such as interactive audio/video signals, over a network.
One way of providing guaranteed QoS communication in an ATM network is to use the Constant Bit Rate, or CBR, service. The CBR service is provided via rate control at sources which limits the maximum rate that data cells can be injected into a network for a connection, and connection admission control (CAC) using peak bandwidth allocation to limit the aggregated bandwidth of all CBR connections over a transmission link not to exceed the link bandwidth. However, rate control and peak bandwidth allocation alone can not automatically guarantee QoS for CBR connections due to traffic distortions within a network. Specifically, due to cell jitter introduced at upstream nodes, cells of a CBR connection may arrive at a switching node in a network at a higher rate than they are generated at the source for a certain period of time. This fluctuation in cell rates may cause aggregated cell rate exceed the link bandwidth, resulting in unpredictable queueing delays for CBR cells. To provide guaranteed QoS for CBR connections, more sophisticated CAC mechanisms than peak bandwidth allocation are needed to calculate worst-case queueing delays for CBR connections and make sure that the worst-case delays do not exceed the delay bounds requested by applications.
In addition, CBR is not always a suitable service for providing real-time communication. Real-time traffic such as encoded video signal has variable bandwidth. It causes over-reservation of network resources if one assigns a connection with a bandwidth equal to the peak rate of the traffic. The Variable Bit Rate (VBR) service as defined by the ATM Forum is more suitable for bursty real-time traffic. The traffic model for a VBR connection is described by a peak cell rate (PCR), a sustainable cell rate (SCR), and a maximum bursty size (MBS). A VBR connection is allowed to inject up to MBS cells into a network at a rate of PCR under a constraint that the average cell rate does not exceed SCR. In other words, a connection is allowed to generate traffic at a higher rate for a certain period of time if it did not use up the average bandwidth allocated to it. The VBR service can support bursty real-time communication better than the CBR service. However, no good CAC mechanisms are known so far to provide hard real-time guarantees for VBR connections. A more detailed description of the CBR/VBR services can be found in a document entitled "ATM Forum Traffic Management Specification Version 4.0" authored by the ATM Forum, April 1995.
In the subject invention, we introduce a bit-stream traffic model to describe the worst-case traffic pattern of a CBR or VBR connection and a mechanism to obtain the worst-case queueing delay based on the model. This worst-case queueing delay is then used for connection admission control.